Recently, Zhang proposed a reversible data hiding scheme for encrypted image with a low computational complexity
which is made up of image encryption, data embedding and data-extraction/image-recovery phases. During the last
phase, the embedded data are extracted according to a determined smoothness measuring function on each nonoverlapping
block. However, not all pixels in a block are considered in his approach. This may cause higher error rate
when extracting embedded data. In this paper, we propose a novel smoothness evaluating scheme to overcome the
problem. Based on the Zhang's approach, we divide the pixels in each block into three different portions: four corners,
four edges, and the rest of pixels. The smoothness of a whole block is determined by summing the smoothness of three
portions and is utilized to extract embedded data and recovery image. Experimental results show that the proposed
scheme can reduce the error rate of data-extraction/image-recovery effectively. For a given normal testing image, such as
Lena, supposing that the size of each block is 8 by 8, the error rate of our approach is less than 0.6% and Zhang's method
is higher than 12%. Moreover, the error rate will be zero when the size of each block is defined as 12 by 12.